Arrest of tumor growth in vivo depends upon (1) the structural integrity of the cAMP receptor protein and (2) the optimum cellular concentration of cAMP. Arginine plus DBcAMP (at a low dose) acted synergistically to enhance the growth inhibitory effect. DBcAMP may act intracellularly to induce cAMP receptor protein, whereas arginine appears to act by increasing the cellular cAMP level via NAD-dependent activation of adenylate cyclase at cell membrane.